The present invention relates to a spectrophotometer and, more particularly to a spectrophotometer which is used as a double-monochromator or a single-monochromator.
The most popular double-monochromator is constructed with two spectroscopes disposed in series, each having a dispersion element and reflection mirrors and is used only as a double-monochromator. On the other hand, a single-monochromator has one spectroscope and some reflection mirrors and is used only as a single-monochromator.
The U.S. Pat. Nos. 3,730,626, 3,927,944 and 4,277,079 and Japanese Utility-Model Laid-open No. 5376978 (1978) are cited as examples of such devices.
But the conventional technique as described above has the following drawbacks. The conventional double-monochromator is generally characterized by little stray light, because it has the two spectroscopes disposed in series. As a typical dispersion element installed in the spectroscope, a diffraction grating or a prism is generally used.
When detecting the light passed through a concentrated sample, the light energy is small, but since the double-monochromator has the two spectroscopes disposed in series as stated above, the stray light is removed effectively and further the purity of the detected monochromatic light becomes higher. Therefore, the signal to noise ratio of the detected light becomes higher of detecting the light passed through a concentrated liquid sample.
But, when detecting the light passed through a liquid sample which is not concentrated, the energy of the light detected by the double-monochromator becomes smaller and more stray light generates so as to make the signal to noise ratio of the detected light lower.
On the contrary, the single-monochromator detects more stray light, but the energy of the detected light increases. So, the signal to noise ratio of the single-monochromator is higher than that of the double-monochromator in the case of detecting the light from the liquid sample which is not concentrated.
Therefore, a user needs to use both functions of the two monochromators, and must own both of a single-monochromator and a double-monochromator, but it is expensive for the user.